1. Field of the Invention
The present invention relates to a CDMA asynchronous cellular radio communication system, particularly a COMA radio communication apparatus for identifying a long code used in a base station.
2. Description of the Related Art
In a CDMA asynchronous cellular radio communication system using a mobile terminal device such as a portable telephone and a car telephone, it is necessary to perform a cell search for the initial synchronization (acquisition) when a mobile terminal device is turned on or for a cell switching as moving (handover).
In forward link control channel signals, a common short code to each cell and a different long code for each base station are used, and the long code has masked intervals in it. The cell search is performed by detecting the correlation of a masked symbol with a common short code in the long code masked interval to detect a timing and a sort of the long code.
In detail, in a CDMA asynchronous cellular mobile communication system, a transmission side (base station) uses a common short code to each cell (CSC) as a code for search (search code) and a long code group identification short code (GIC) corresponding to the group to which a long code for each cell belongs, and transmits a frame in which a symbol spread with CSC and a symbol spread with GIC are code multiplexed in a plurality of long code masking intervals in slots prepared at equal periods in a frame (for instance, each slot).
A reception side (mobile terminal device) first detects a timing of a slot using a common short code to each cell. Then it defines the candidates of long codes to search using a long code group identification short code, further specifies a long code specific to the cell from the candidates of long codes.
FIG. 1 is a block diagram illustrating a configuration of a conventional initial synchronization section in a radio communication apparatus (mobile terminal device) in a CDMA asynchronous cellular mobile communication system. A signal transmitted from a base station is received in a mobile terminal device as input signal 20. Input signal 20 is processed in matched filter 3 to detect the correlation with a common short code to all base stations generated in common short code generating section 1.
The output in matched filter 3 is converted into a power value in electric power converting section 4, and the power value is averaged in averaging section 5. The power value of the correlation necessary to average is stored in memory 2. In the prescribed period, the maximum value among the averaged power values is detected in maximum value detecting section 6, and a timing detected in masked symbol timing generating section 7 is a masked symbol timing.
On the other hand, input signal 20 is processed in correlator 8 to sequentially detect the correlation with a long code group indication short code sequentially generated in long code group indication short code generating section 12 at the masked symbol timing. Each of the outputs in correlator 8 is converted into a power value in electric power converting section 9, and the power values obtained in the prescribed period are integrated in integrating section 10. Next the maximum value among the integrated power values is detected in maximum value detecting section 11, and by using a long code group identification short code with the maximum value, a long code group is identified. In addition, the output in maximum value detecting section 11 is transmitted to long code timing controlling section 21.
And input signal 20 is processed in correlator 15 to detect the correlation with a long code and short code generated in long code/short code generating section 19. For a long code/short code generated in long code/short code generating section 19, a phase of the long code is varied in for instance, slot interval in long code timing controlling section. And the long code is sequentially selected from the long codes classified in the long code group identified as described previously.
The output in correlator 15 is converted into a power value in electric power converting section 16, and the power values obtained in the prescribed period are integrated in integrating section 17. Next threshold value deciding section 18 decides whether or not the integrated value exceeds the threshold value, then identifies the long code with the value exceeding the threshold as a long code of the base station, and identifies the timing of the slot as a phase of the long code. In other words, the correlation detection is processed by varying a phase corresponding to the number of slots while by varying a long code classified in the long code group until the integrated value of correlation of a common short code to all base stations exceeds the threshold value. Thus a cell search is performed.
In a conventional initial synchronization section, however, in a CDMA radio communication apparatus described above, in the process to identify a long code from a long code group, since a phase differs depending on the number of slots, it must be performed to detect a long code phase along with the identification of a sort of long code. Accordingly there has been the problem that the problem it takes much time to acquire the identification of a long code, e.g., the initial acquisition. On the other hand, in the case of processing the long code identification in parallel in the hardware, the long code identification time can be reduced. However it brings the problem that the large scale of hardware is necessary.